In drilling holes in materials such as concrete and stone, a special drill bit is attached to a drilling machine, e.g., a rotary hammer drilling machine, and either cutting by rotation or drilling by action of simultaneous application of a vibrational striking force in axial direction and a rotational torque is carried out. And, in order to meet the demand for higher efficiency in such a type of drilling work, drill bits of the type having a cutting blade tip of cemented carbide superior in resistance to abrasion which is united tightly to a tip end side of a steal bit body of the drill bit by brazing, welding, or the like, have been used extensively. Further, like a general-purpose drill, the following configuration has been employed in many cases. That is, a standardized shank portion conforming to the drive shaft is extendedly formed in a base portion of the bit body, and chip ejection grooves are formed in the outer peripheral surface of the bit body in association with cutting blade portions of the cutting blade tip, respectively.
On the other hand, during drilling operations against materials such as concrete and stone by use of the drill bit, i.e., during drilling work (cutting work), lots of powder/particle-like chips are created. However, these cutting chips are not always ejected out through the chip ejection grooves of the bit body.
Particularly, when drilling a hole using a rotary hammer drilling machine or a vibration drilling machine, cutting (impact cutting), in which the blade tip of a drill bit momentarily surfaces from a drill hole cutting surface and immediately thereafter starts impactingly knocking the drill hole cutting surface, is carried out. And, the movement of the drill bit in the longitudinal direction of the shaft of the drill bit during such knocking action also helps a lots of chips to be ejected out through a clearance defined between the bit body and the drill hole.
Further, for the case of rotary hammer drilling machines or for the case of vibration drilling machines, even when chips of relatively large particle size are created on a drill hole cutting surface, they are ground into small particles while impact cutting is repeatedly carried out on the drill hole cutting surface. Therefore, the situation that chips of large particle size remain in a drill hole to cause interference with drilling operations hardly occurs.
However, for the case of drill bits of small bit diameter, e.g., for the case of a drill bit whose bit diameter is not more than about 6 mm and which is provided with chip ejection grooves formed helically around the outer peripheral surface of a drill body, the rigidity of the drill bit falls because of the formation of the chip ejection grooves. This increases the rate of breakage during drilling operations. Particularly, for the case of drilling operations with a rotary hammer drilling machine or with a vibration drilling machine in which impact force in the longitudinal direction of the shaft is applied, the rate of breakage becomes high.